Disclaimer

I disclaim everything. The contents of the articles below might be totally inaccurate, inappropriate, or misguided. There is no guarantee as to the suitability of said circuits and information for any purpose whatsoever other than as a self-training aid.

Light dimming is based on adjusting the voltage which gets to the lamp.
Light dimming has been possible for many decades by using adjustable
power resistors and adjustable transformers. Those methods have been
used in movie theatres, stages and other public places. The problem of
those light controlling methods have been that they are big, expensive,
have poor efficiency and they are hard to control from remote location.

The power electronics have proceeded quickly since 1960. Between 1960-1970
thyristors and triacs came to market. Using those components it was
quite easy to make small and inexpensive light dimmers which have good
efficiency. Electronics controlling also made possible to make them easily
controllable from remote location. This type of electronic light dimmers
became available after 1970 and are nowadays used in very many locations
like homes, restaurants, conference rooms and in stage lighting.

Solid-state light dimmers work by varying the "duty cycle" (on/off time)
of the full AC voltage that is applied to the lights being controlled.
For example, if the voltage is applied for only half of each AC cycle,
the light bulb will appear to be much less bright than when it
get the full AC voltage, because it get's less power to heat the filament.
Solid-state dimmers use the brightness knob setting to determine at what point in each voltage cycle to switch the light on and off.

Typical light dimmers are built using thyristors and the exact time
when the thyristor is triggered relative to the zero crossings of the
AC power is used to determine the power level. When the the thyristor
is triggered it keeps conducting until the current passing though it
goes to zero (exactly at the next zero crossing if the load is purely
resistive, like light bulb). By changing the phase at which
you trigger the triac you change the duty cycle and therefore
the brightness of the light.

Here is an example of normal AC power you get from the receptacle
(the picture should look like sine wave):

As you can see, by varying the turn-on point, the amount of
power getting to the bulb is adjustable, and hence the light
output can be controlled.

The advantage of thyristors over simple variable resistors is that they
(ideally) dissipate very little power as they are either fully on or fully off.
Typically thyristor causes voltage drop of 1-1.5 V when it passes the
load current.

What are thyristors and triacs

A Silicon Controlled Rectifier is one type of thyrister used where the
power to be controlled is unidirectional. The Triac is a thyrister
used where AC power is to be controlled.

Both types are normally off but may be triggered on by a low current
pulse to an input called the gate. Once triggered on, they remain on
until the current flowing through the main terminals of the device
goes to zero.

Both SCRs and Triacs are 4 layer PNPN structures.
The usual way an SCR is described is with an analogy to a pair of
cross connected transistors - one is NPN and the other is PNP.

If we connect the positive terminal of a supply to say, a light bulb, and
then to the emitter of the PNP transistor and its return to the emitter
of the NPN transistor, no current will flow as long as the breakdown
voltage ratings of the transistor are not exceeded because there is no base
current to either.

However, if we provide some current to the base
of the NPN (IG(+)) transistor, it will turn on and provide current to the base
of the PNP transistor which will turn on providing more current to
the NPN transistor. The entire structure is now in the on state and
will stay that way even when the input to the NPN's base is removed
until the power supply goes to 0 and the load current goes to 0.

The same scenario is true if we reverse the power supply and use the
IG(-) input for the trigger.

A Triac works basically in a similar manner but the polarity of the
Gate can be either + or - during either half cycle of an AC source.
Typically the trigger signals used for triggering triacs are
short pulses.

Incandescent lamp physics

A typical incandescent lamp take power and uses it to heat up a filament
until it will start to radiate light. In the process about 10% of
the energy is converted to visible light.
When the lamp is first turned on, the resistance of the cold filament
can be 29 times lower than it's warm resistance. This characteristic is
good in terms of quick warmup times, but it means that even 20 times the
steady-state current will be drawn for the first few milliseconds
of operation.
Lamp manufacturers quote a typical figure for cold lamp resistance of 1/17 th of
the operational resistance, although inrush currents are generally
only ten times the operational current when such
things as cable and supply impedance are taken into account.
The semiconductors, wiring, and fusing of the dimmer
must be designed with this inrush current in mind.
The inrush current characteristic of incandescent (tungsten filament)
lamps is somewhat similar to the surge
characteristic of the typical thyristors made for power controlling, making
them a quite good match. The typical ten times steady
state ratings which apply to both from a cold start allow many triacs to
switch lamps with current ratings close to their own steady state ratings.

Because lamp filament has a finite mass, it take some time
(depending on lamp size) to reach the operating temperature
and give full light output. This delay is perceived as a "lag",
and limtis how quicly effect lighting can be dimmed up. In theatrical
application those problems are reduced using preheat (small current
flows through lamp to keep it warm when it is dimmed out).

The ideal lamp would produce 50% light output at 50% power input.
Unfortunately, incandescents aren't even close that. Most require at
least 15% power to come on at all, and afterwards increase in intensity
at an exponential rate.

To make thing even more complicated, the human eye perceives light intensity
as a sort of inverse-log curve. The relation of the the phase control value
(triac turn on delay after zero cross) and the power applied to the
light bulb is very non-linear. To get around those problems, most
theatrical light dimmer manufacturers incorporate proprietary
intensity curves in their control circuits to attempt to make selected
intensity more closely approximate perceived intensity.

Very basic circuit

This is the type of common light dimmer widely available at hardware
stores and home centers. The circuit is a basic model for light
dimmer for 120V AC voltages. This basic design can handle light bulbs
at power range of around 30W to few hundred watts (depends on construction).

The purpose of the pot P1 and capacitor C2 in a diac/triac combination is
just to delay the firing point of the diac from the zero crossing.
The larger the resistance (P1+R2) feeding the capacitor C2, the longer it takes
for the voltage across the capacitor to rise to the point where the
diac D1 fires turning on the triac TH1. Capacitor C1 and inductor L1
make a simple radio frequency interference filter. Without it the
circuit would generate quite much interference because firing of
the triac in the middle of the AC phase causes fast rising current surges.
The triac TH1 can withstand 6A of continuous current when properly cooled, so
the circuit would be able to handle around 300-500W of power when a small
heatsink is fitted to TH1. If TH1 is not cooled, the maximum power
rating is probably around 150W.

While the dimmer is designed for incandescent or heating loads only, these
will generally work to some extent with universal motors as well as fluorescent
lamps down to about 30 to 50 percent brightness. Long term reliability is unknown for
these non-supported applications.

Minimal circuit

I also saw a quite similar dimmer circuit posted to sci.electronics.design
newsgroup one day (posted by Sam Goldwasser).
This is the type of common light dimmer (e.g., replacements for standard
wall switches) widely available at hardware stores and home centers.
This circuit uses slightly different component values than the previous one
and does not have any radio frequency interference filtering.
This one contains just about the minimal number of components to work at all!

S1 is part of the control assembly which includes R1.
The reostat, R1, varies the amount of resistance in the RC trigger circuit.
The enables the firing angle of the triac to be adjusted throughout nearly
the entire length of each half cycle of the power line AC waveform. When
fired early in the cycle, the light is bright; when fired late in the cycle,
the light is dimmed.

The circuit should be able to handle loads up to aorun 150W without
a heatsink. If a large heatsink is provided for TH1, the circuit should
theoretically be able to handle loads up to almost 1 kW, but I would
not try more than 800W.

Due to some unavoidable (at least for these cheap
dimmers) interaction between the load and the line, there is some hysteresis
with respect to the dimmest setting: It will be necessary to turn up the
control a little beyond the point where it turns fully off to get the light
to come back on again.

Brief description of circuit of circuit operation:
The delay from mains zero crossing to triack triggering is generated using
circuit formed with R1, C1 and diac.
The adjustable resistor R1 resistance controls the speed at which
C1 charges from incoming power. Higher the resistance,
longer it takes C1 to charge to the specific voltage.
When the voltage at C1 goes to the trigger voltage (usually around 30V)
of the diac, the diac starts to conduct, which discharges
the charge from C1 through diac to the triac gate causing
it to trigger. The result of this the voltage at C1 goes
to zero volts (very near to it), and the triac starts to conduct.
Triac conducting causes power to flow though the circuit to
the load (light bulb). The voltage over triac is almost zero
(in practice around 1V or less), so the capacitor does not get
charged as long as the triac conducts.
The triac conducts as long as there is enough current flowing though it,
in this case until to the next mains voltage zero crossing.
At that point the operation starts again from charging of C1.

The following circuit is HELVAR 1 kW light dimmer dimmer circuit
published at Bebek Electronics magazine.
The circuit is a quite typical TRIAC based dimmer circuit with no fancy special features.
The triggering circuit is a little bit improved compared to the 120V AC above design.
This circuit is only designed to operate with non-inductive loads like standard
light bulbs. The circuit is designed to dim light bulbs in 50-1000W range.

Potentiometer P1 in this circuit is used for controlling the dimmer setting.
The trimmer P2 is used for setting the dimming range (how much light can
be dimmed maximally). When the circuit is tuned, the P2 should be
adjusted so that then P1 is in it's maximum resistance setting
(light most dimmed) the light bulb is just dimmed completely out.
This adjustment makes sure that the dimmer circuit dims smoothly
from zero to maximum setting. If P2 is tuned to too much dimmed preset
position, the circuit does not dim nicely up from light off setting
or the operation when P1 is in it's maximum value is unpredictable.
If you have adjusted P2 to too low value, you just can't dim the
light bulb completely off (in some times this can be an intentional
setting, for example in theatrical lighting where preheat is used).

When building thw circuit remeber to put a small heatsink to
the triac TH1, because without proper cooling it can't withstand
the full dimmer 1 kW power (around 4.4A of current). If you don't
put the heatsink, the maximum available power from the circuit is
around 300W. The coil L1 must be able to withstand continuous current
of at least 4.5A and it can have any value between 40 and 100
microhenries. For C1 I would recommend a good quality 150 nanofarad
capacitor designed for mains power applications (propably an X-rated capacitor),
because a low quality capacitor does not withstand in this kind
of place for too long time.

Because light dimmers are directly connected to mains you must make sure
that no part of the circuit can be touched when it is operating. This
can be best dealt by building the dimmer circuit to small plastic box.
Remeber to use potentiometer with plastic shaft and install it so that
no potentiometer metal parts are exposed to user.

Remeber to make circuit board so that the traces have enough
current carrying capacity for the maximum load. Make sure that you
have enough separation between PCB traces to widthstand mains voltage.
Remeber to install correct size fuse for the circuit. The fuse shield be
ast acting (F) if you want to give any protection to TRIAC
(do not use FF or T types). Make sure that all components
can handle the voltages they face in the circuit. For 230V operation
use at least 400V triac (600V better). The capacitor which is connected
between the dimmer circuit mains wires should be a capacitor which
is rated for this kind of applications (those are marked with letter X on the
case).

Remeber to use coil type which can handle the full load current without
overheating or saturating. Use capacitors with enough high voltage
rating. Make sure that the TRIAC has enough ventilation so that it does
not overheat at full load. For safety reasons it is a very good idea
to put an overheating protector to the light dimmer circuit to
protect the dimmer circuit against dangerous
overheating caused by poor ventilation or slight overloading,
because a fuse does not provide a good protection in this kind of cases.

Even though the light can be completely turned off using triac or
thyristors, those components are not generally considered to be
reliable enough to be used as light switches which remove the
dangerous voltages from the light circuit when needed. In small
light dimmer there is typically a switch which is built into the
light dimmer control potentiometer. In large dimming systems the
switching is typically done using a separate contactor or relay.

Triacs and thyristors are sensitive to overcurrents.
When dimming normal light bulbs, short circuits caused when
filament burns are quite probable. For this reason, light dimmers
must have their own fuse which protect it against failures in
this kind of situation.

Thyristors have a defined overcurrent handling capacity and the
fuse must be selected so that it burns before the thyristor
in overcurrent situation. This typically means that the thyristor/triac
must have a current rating of 2..5 times bigger that the rating
of the fuse in order to be sure that the fuse burns before thyristor/triac
in case of short circuit. The fuse type must be also fast enough to burn
in this case before the thyristor/triac. In some cases it might be necessary
to use special fuses to be able to protect the components effectively.

The thyristor must have a high enough surge current rating also for
normal operation. For example in case of normal light bulb dimming of
a light bulb with cold filament is turned on at 90 degrees after
zero crossing (means at maximum line voltage peak), the peak current
can be 20 times bigger than the nominal current of the lamp.

The modern thyristor (Triac or SCR) dimmer has one fairly severe
drawback in its performance in that it dims by
switching on the current to the load part-way through each mains
cycle. Cutting the leading smooth-part off a mains
cycle produces a current with a very rapid turn-on time which
generates both mains distortions and EMI. Chokes are
included in dimmers to slow down the rapid switch-on (rise time) of
the chopped current. The longer the rise time the
less EMI and mains distortion produced.

Turn on of the triac in the middle of the phase
causes fast voltage and current changes. A typical thyristor/triac
starts to fully conduct at around 1 microsecond time
after triggering, so the current change
is very fast if it not limited in any way. Those fast voltage and current
changes cause high frequency interference going to mains wiring unless
there are suitable radio frequency interference (RFI) filter built into
the circuit. The corners in the waveform effectively
consist of 50/60Hz plus varying amounts of other frequencies that
are multiples of 50/60Hz. In some cases the interference goes up to
1..10Mhz frequencies and even higher. The
wiring in your house acts as an antenna and essentially
broadcasts it into the air.
Cheap bad quality light dimmers don't have adequate filtering
and they cause easily lots of radio interference.

Dimmer circuits typically use coils that limit
limit the rate of rise of current to that value which
would result in acceptable EMI. Typical filtering in
light dimmers causes the current rise time
(current rises from 10% to 90%) to be in range of 30..50 microseconds.
This gives acceptable results in typical dimmer applications in home
(typically this limitation is made using 40..100 uH coil).

If the dimmers are used in places where dimmer is a serious problem
for sensitive sound equipments (theatres, TV-studios, rock concerts etc.)
a slower current rise time would be preferred.
Typically the current
rise time in light dimmer packs made for stage applications have
a current rise speed of around 100..350 microseconds. If noise
is a big problem (TV studios etc.), even slower current rise times
are sometimes asked. Those current rise times up to 1 millisecond can be
achieved with special dimmers or suitable extra coil fitted in series
with the dimmer.

The coil itself does not typically
solve the whole problem because of the self-capacitance of the inductor:
they typically resonate below 200 kHz and look like capacitors to
disturbances above the resonance frequency. That's why there must
be also capacitors to suppress the interference at higher frequencies.

If your dimmer circuit cause interference, you can try to filter out the
interference by adding a small capacitor (typically 22nF to 47 nF) in parallel
with the dimmer circuit as near as possible to the electronics inside the
circuit as possible. Keep in mind to use a capacitor which is rated for this
kind of applications (use capacitors marked with X). Keep in mind that the
filter capacitor and it's wiring make a resonance circuit with certain
resonance frequency (typically around 3.6 MHz with 0.1 uF capacitor).
The capacitor does not work well as filter with the frequencies higher than
the resonance frequency of the circuit.

All phase control dimmers are non-linear loads.
A non-linear load is one where current is not in proportion to voltage.
The non-linear load on dimming systems is caused by the fact that
current is switched on for only part of the line cycle by a phase control dimming system.
This non-linear load creates harmonic distortion on the service feeder.

Harmonics are currents that occur at multiples of the power line voltage frequency. In Europe where line frequency is 50 Hz the
2nd harmonic frequency is 100 Hz; the 3rd harmonic is 150 Hz, and so on.
In North America where line frequency is 60 Hz the 2nd harmonic frequency is 120 Hz; the 3rd harmonic is 180 Hz, and so on.

Excess harmonic currents cause conductors and the steel cores of transformers and motors to heat. Odd-order harmonic currents (specifically the 3rd harmonic) add together in the neutral conductor of 3 phase power distribution systems.
The 3rd order harmonic current present on the neutral is the arithmetic sum of the harmonic current present on the three phase conductors
(this also applies to the 9th, the 15th and so on harmonics).
Harmonics could theoretically elevate the neutral current to 3.0 times what is present on a phase conductor. With typical phase control dimming system
connected to three pahse feed, the harmonics normally elevate neutral
current to about 1.37 times phase current.
If the wires are not properly rated for this, neutral conductor overheating
or unexplained voltage drops can occur in large dimming systems.

Sometimes the heating of the distribution trasformer can be a problem,
because transformers are rated for undistorted 50 Hz or 60 Hz load currents.
When load currents are non-linear and have substantial harmonic content,
they cause considerably more heating than the same undistorted current.
In heavily dimmed system, you might not be able to ultilize more than
around 70 % of the rated transformer power rating because of harmonic
induced heating. Additionally, transformers used to feed dimming systems
are subjected to stress because of cold lamp inrush currents
(can be up to 25 times normal current). Inrush currents and harmonics
can drastically reduce the service life of the service transformer.

Eliminating the effects of harmonic currents in large light dimmer
systems normally requires oversizing neutral conductors and derating
the service transformer.

In a normal low power light dimmer case you don't have to woryr
much about the harmonics and transformer loads, because the
light load of few hundred watts is clearly just a small fraction
of the total transformer load.

Each good dimmer has a filter choke inside.
Those chokes help to filter out electrical noise that often causes hum
to be picked up in sound system and musical instrument pick-ups.
The slower the current rise is, the less noise is picked by sound system.

The chokes also help to eliminate 'lamp singing' that can cause
audible noise to come from the lighting fixtures. Lamps with power rating
of 300W or more tend to more or less acoustic noise when dimmed.
If this acoustic noise is a problem can be removed by adding a series
coil which limits the current rise time to around 1 millisecond.

In providing those filtering functions, the chokes themselves
can generate a slight buzz.
Fast current changes in the coil can make the coil wiring
and core material easily vibrate which causes buzzing noise.
A little bit of buzzing is normal with filtered dimmers.
If the buzz from dimmer can be a problem it is recommended
that the dimmer is placed in the area where this buzz will not be a problem.

As far as the 'bulb singing' concerned, a bulb consists of a series
of supports and, essentially, fine coils of wire. When the
amount of current flow abruptly
changes the magnetism change can be much stronger than it is on
a simple sine wave. Hence, the filaments of the bulb will tend
to vibrate more with a dimmer chopping up the wave form, and
when the filaments vibrate against their support posts, you
will get a buzz. If you have buzzing, it's always
worth trying to replace the bulb with a different brand. Some
cheap bulb brands have inadequate filament support, and simply
changing to a different brand may help.

Buzzing bulbs are usually a sign of a "cheap" dimmer. Dimmers
are supposed to have filters in them. The filter's job is to
"round off" the sharp corners in the chopped waveform, thereby
reducing EMI, and the abrupt current jumps that can cause
buzzing. In cheap dimmers, they've economized on the
manufacturing costs by cost-reducing the filtering, making it
less effective.

In very high power dimming systems the wiring going to lighting can also cause
buzzing. The fast current makes the electrical wiring to vibrate a little
bit and if the wire is installed so that the vibration can be transferred to
some other material then the buzzing could be heard. The buzzing caused
by the vibration of the wiring is only problem in very high power
systems like theatrical lighting with few kW of lights connected to
the same cable. Better filtered dimmers can reduce the problem because
the filter makes the current changes slower so the wires make less noise.

Why does dimmed lighting sometimes hum, and how can it be corrected?

Because of the way all dimmers deliver power at settings other than full brightness, the
filaments inside a light bulb may vibrate when lighting is dimmed. This filament vibration
causes the hum. To silence the fixture, a slight change in the brightness setting will usually
eliminate bulb noise. The most effective way to quiet the fixture is to replace the light bulb.

There are numerous ways that dimmer noise can get into audio systems and
it's largely trial and error in determining what in particular is causing
your problem and hence how to fix it.
The principle ways are either back up the mains
or induced into your audio equipment or cables.

What you hear typically in audio system is
common mode noise on the hot and neutral, the spike of turn-on of the scr.
The higher the rise time of the current in the dimmer, more noise
is sent to the mains wiring. So well filtered dimmer will generate
less noise problems.

Reduce the possibility of it coming up the mains by taking a totally
separate mains supply from the lighting, if possible get a totally
separate power socket (or sockets) run in for sound from wherever the
electricity board intake is. If this is not possible, then
an isolation transformer stops quite much of
the noise on the secondary side (better with shield between coils).
So put the sound system on the isolation transformer and tie to earth
(ground) almost no problems. This assume that sound wiring is
correct, especially shielding is done well and ground loop are avoided.

To reduce the possibility of interference induced to the audio cables,
run all non speaker level audio cables as balanced lines
(or certainly all of any length).
You might have to buy balancing transformers if your kit
isn't balanced already. Also keep them as far away physically from any
lighting cable runs as you can. Make sure that your system does
hot have any harmful ground loops.
Make sure none of your audio kit is anywhere near the dimmer racks.

Now can I dim up the lights smoothly ?

With many cheap dimmers, the lights "Pop On" rather than dim up smoothly.
This problem is usually related to the construction of the dimmer
electronics. One technique used in some cheap dimmers to allow
dimming up smoothly is to place another potentiometer (trimmer)
across the control potentiometer. That trimmer potentiometer
is set so that the dimmer works smoothly:

a)Set "Control" to Minimum light level.

b)Adjust "Trimmer" to filaments JUST "glow"

c)Turn off dimmer

d)Turn on dimmer to see if filaments "glow".
IF not... set trimmer up a snit.... go to c)

Continue until minimum voltage/current is supplied to lamps
(filaments do not seem to glow at all).
When everything is properly adjusted, the dimmer circuit will
nicely dim up from the lowest setting up to maximum brightness.

Are those household dimmers usable as stage lighting dimmers ?

If you want to make a multichannel lighting desk, you might sometimes
winder if such nit can be built from cheap household dimmers.
Unfortunately most cheap household dimmers are no use for stage lighting.
The limitations in this kind of use came from performance,
power rating, reliabity and interferences.

Typically the cheapest dimmer won't fade up
smoothly from zero, but come on suddenly at about 20%. You can fade down
smoothly, but once they go off you have to go back up to 20% to make them
come on. There are some dimmers which perform better that other.

The cheapest household dimmers are typically not well filtered, so
the interference caused by a multichannel dimming board built
in this way can easily cause a sound system to buzz.

Then in many cases the power rating of household dimmers
can be a problem. Usually the household dimmers have a power
rating of around 300W, which is not enough for any powerful
stage light which can easily be 500W in power.

Cheap household dimmers do not track with each other well.
This means that at the same
setting, the lamps on one circuit will appear to be twice as bright as those
on the other circuit.

Normal light dimmers are designed to only dim non-lunductive loads like
light bulbs and electric heaters. Normal light dimmers are not suitable
to dim inductive loads like transformers, fluorescent lamps, neon lamps,
halogen lamps with transformers and electric motors. There are
special dimmers available for those applications.

If you connect inductive loads to the dimmer the dimmer might not
work as expected (for example does not dim that load properly)
and can even be damaged by the voltage surges generated by the
inductive load when current changed radiply.
Another problem is the phase shift between the voltage and
current cause by the inductance. If you use a normal
simple light dimmer which is just in series with the wire
going to the load, this will cause that the dimmer circuit will not
wirk properly with highly inductive loads. Special dimmers
which have a separate controlling electronics connected to both
live and neutral wire and then the triac which controls the current
to the load usually work much bettter with inductive loads.

Often when inductive loads cause problems on normal dimmers, you can
eliminate said problems by patching an incandescent "ballast" load
in parallel with the inductive load. Usually 100W is enough for
many inductive loads. Remeber that indictive loads can hum quite
noticably when dimmed and the transformers can heat more because
of increased harmonics content in the power coming to them.

Dimming lights with built-in transformers

Fully loaded halogen transformers usually dim quite well.
If you are planning to dim halogen light transformers, try only
dim traditional transformes, because toroidal core transformer do not
usully dim well. Most of the cheap halogen light transformers
belong to this category as well as the transformer in for example
PAR36 pinspot lights. For this kind of transformer it is necessary
that the current after the dimmer is still symmetric, so that
there is no DC component formed to the transformer which can
cause the transformer cire to aturate (and lead to overload
and finaly destruction of transformer). Some of the cheapest
light dimmers might not be very good on symmetry,
but good quality light dimmers designed for also inductive
loads should not have symmetry problems.

When dimming transformers with in any way questionable
type do dimmer for inductive loads, it is a good idea
to put a fuse in series with the transformer primary so that it will
blow when transfromer tries to get too much power from the line.
This will protect the transformer from overheating which might be caused
because of transformer core saturation (which might be caused by small
DC bias caused by not very well operating dimmer). A proper fuse
will save transformers from burning out.

Anyway a normal transformers which feed light loads are
dimmable with good quality dimmer which can handle at least
some amount of inductive load usually without much problems.
Anyway it should be mentioned that when a transformer is
dimmed in this way, it can heat somewhat more than in normal
operation (full power without dimming). Other thing
worth to mention is that when a tranformer is dimmed,
it usually produces noticably more audible noise than
in normal operation (noise depends on used transformer).

If your halogen light system uses an electronic transformer
then you must very carefully check if it can be dimmed.
Some of the electronic transformers are made dimmable and work
well with traditional light dimmers. The ones which are not ment to
be dimmed can be damaged by the dimming and even damage your dimmer.

Dimming fluorescent lights

If you try to
dim fluorescent light on normal dimmer you have to turn the dimmer
full on to make the light to turn on and you can only dim it down
only down to 30-50% brightness. For anythign less than this you will
need a special dimmers and special fluorescent fitting.

Dimming electric motors

Typical dimmer packs will supply power to motors and make them run, but
the dimmers aren't designed for it. Some dimmers can be damaged by connecting
inductive loads to them. And when the triac fails half-wave it
takes the motor out too. A good idea to protect motor failures
is to use a fuse sized for the motor load in series with the motor.
This fuse will propably burn before motor is damaged if it is sized
correctly.

Light dimmers designed for inductive loads work quite well
with universal" or AC/DC type motors. Typically, these have
brushes and are used in electric drills, vacuum cleaners,
electric lawn edgers etc. With this kind of
motors a proper dimmer works well.

The motors used in electronics fans are quite likely
induction motor which are not very well controllable.
Those motors in most fans are square-law
devices, most of the speed control will be at the end of the dial but that
would be true with any control. The "dimmers" designed for ceiling
fan speed control work quite well and also some normal light
dimmers designed for inductive loads.

If the dimmer approach not satisfactry, then remeber that
electric motors are usually is best controlled by a small variac, tapped
ransformer, rheostat, series light bulbs, etc. which do not mess up the
sinusoidal waveform. Even this method does not help in controlling
a syncronous motor, which always tries to rotate at the same
speed suncronous to mains power.

Electronic loads like switching power supplies are not generally
designed to be dimmed. If you take for example a typical
swithcing power supply to a normal light dimmer, trying to
do that might result to cause damages to the dimmer and/or the
power supply itself. The power supply might get damaged because
it has never been designed to operate on other waveforms
than quite much sinewave (other waveforms can cause current
spikes). The dimmer can be damaged by the high current surge what
a switching power supply takes when the triac on dimmer starts
to conduct in the middle of the phase.

The "electronic transformers" used to power the 12V halogen lamps which are very
fashionable for indoor lighting. Those "transformers" are small
swithcing power supplies which just chop the mains at about 40kHz, so a small
ferrite core can be used for the isolation and the voltage step-down
(to 12V RMS).

Generally it not a good idea to try to connect this kind
of "transformer" to a normal light dimmer unless that
"transformer" is a type which is designed to operate
correctly with a normal light dimmer (in that case the
fact is said on instructions of the "transformer" or it's case).
There are for exapmle some small transformers available
which say "dimmable with normal light dimmer", so those
can be used without any problems with normal light dimmers.

Other "electronic transformers" I would not try to dim with
a normal phase controlling light dimmer to avoid possible
equipment damages. Quite many electronics transformers (but not all)
which can't be dimmed with normal light dimmer can be dimmed
with transistor based reverse-phase type dimmers. I have
read success tories on this, but never tried this method myself.
If you are planning to use this method, then it is best to
check that the electronic transformers you have dim nicely
and you have a right kind of dimmer for them.

Some of the more expensive "transformers" incorporate a very neat
dimmer functions also, operated by external controls, so with those
there is no need for any external dimmer (just controls).

The basic dimmer operation principle is the same as in dimmers above.
The only difference is how the dimemr is controlled. The rouch controlling
is done using a special control IC and touchable metal plate.
The dimmer usually has a metal plate which is coupled to the circuit
via a high value resistor (>1Meg Ohm). Your body acts a little like an
antenna and couples 50Hz mains signal (or 60 Hz depending on country)
into the circuitry. The AC signal is fed to a shaping
circuit(converted to a square wave) and then usually into a dimmer IC.

A typical touch dimmer has following circuit parts:

A special timing circuit which senses if the contact on the touch plate was long or brief. In operation, a momentary touch of the sensor plate with the fingers (50 - 400 ms) will toggle the light ON or OFF depending on its previous state.

A memory circuit which stores the intensity level of the lights.

A circuit which generates the pulses necessary to vary the light intensity

Touch dimmers which typically control the TRIAC in a 45°C to 152°C conductivity region
of the mains half period while the IC draws its power from the remaining power up to the 180°C of the half period.

Siemens is one of the companies who supply these IC's (for example SLB-0586).
The IC itself will function differently depending how long you touch
the plate for.

Lighting dimmers use
phase-control - you switch on at a point on the supply voltage waveform
after the zero-crossing, so that the total energy input to the lamp is
reduced. The time between zero crossing and switching is controlled by
external control interface which is most often 0-10V DC control voltage
or digital DMX512 interface.

This circuit can control loads up to 2A (460VA).
The circuit is basically a normal light dimmer circuit, but the
potentiometer is replaced with LDR resistor which changes it's resistance
depending on the light level. In this circuit a LED powerred from control
voltage source is used for shining variable intensity light to the LDR, so
you must make sure that LDR does not receive light from other sources.

This circuit is basically very simple and not very sensitive on what LDR is
used as R2. The disadvantage of this circuit is that the control is not
very linear and the different dimmers built around this circuit can have
quite varying characteristics (depending mainly on the LED and LDR
characteristics). The control voltage is optically isolated from
the dimmer circuit connected to mains. If you need a safety solation
then remeber to have enough distace between the LED and LDR or use
a transparent isolator between them to guarantee good electrical isolation.
If the dimmer sensitivity is not suitable with the circuit described above,
then you can adjust the value of R1 to get the control voltage range you
want.

This circuit is a part of an automatic light dimmer circuit published
in Elektor Electronics Magazine July/August 1998 issue pages 75-76.

Professional voltage controlled dimmers

Remotely controlled light dimmers in theatrical and architechtural
applications typically use 0-10V control signal for controlling the lamp brightness.
In this case 0V means that the lamp is on and 10V signal means that the lamp
in fully on. A voltage between those values adjust the phase when the TRIAC will
fire. Here is a typical control circuit schematic:

The circuit works so that the comparator output in low when the input voltage is higher
than the ramp voltage. When the ramp signal voltage gets lower than the input voltage
the comparator output goes high which causes that curresnbt sarts to flow through resistor
to optocoupler which causes the triac to connect. Because the ramp signal starts
at every zero crossing from 10V and goes linearly to 0V at the time of one half cycle
the input voltage controls the time when the triac is triggered after every
zero crossing (so the voltage controls the ignition phase. The necessary linear ramp
signal can be generated by a circuit which discharges a capacitor at constant current
and charger it quickly at every zero crossing of mains voltage.

You can use your own circuit for triggering the TRIAC or you can use
a ready made semiconductor relay for this (it comes in compact package and
provides optoisolation in same package with TRIAC). If you plan to usre
ready made solid state relay you need an SSR WITHOUT zero-crossing switching.
You need an inductor in series with the switching element (SSR ot triac)
to prevent di/dt problems and help to cut down emission of r.f. noise.
Values vary typically from 40 uH to 6 mH: they are usually specified in
terms of the rise-time of the switch-on edge. Typical home light dimmers
use coil of 40..100 uH, whigh gives 30..50 microsecond rise time.
Larger coil values give longer rise time values.
Note that the rise time approximation only rough because the inductors used
are non-linear: the inductance varies with load current.

The optocoupled TRIAC triggering circuit can be for example constructed using
MOC3020 optodiac and some other component. Here is one example circuit
(part of dimmer circuit from Elektor Electronics 302 circuits book):

Most professional stage-ligting dimmers do use solid state relays.
They have more in
them than you would expect, usually including opto-isolation of the
control input. The exact contents are commercially confidential but
the operation of voltage controlled version is very similar to the
idea described above.

Many professiona light dimmer have also extra adjustments
available for make them work better in their operating environment.
One typical setting is cause preheat. When preheat is used a small
(adjustable) current is always passed thought the light bulbs eve thought
the light channel is set off at the lighting desk. This preheat current
keeps the lamp filaments warm (but not warm enough to give considerable light output)
so that the current surge when lights are turned on again is
rediced. This reduced current peak increases the life of the light bulbs.

Another adjustment available in some dimmers is response speed settting.
A dimmer's response speed is the time it takes for the dimmer's
outptu to arrive at a new level after it receives the new level setting
instruction from the control desk. This time is typically measured in milliseconds.
Typical response speeds available on dimmer products are in range of
30..500 milliseconds. A fast response speed is useful in light effects and
concert lighting. In studio uses the light need not typically have to change
very rapidly, so it might be a nice thig if dimmer goes slowly from old
setting to new value. A slower response speed have beneficial effects on
lamp life, since the shock to cold filaments will be reduced, as the time period
required to ramp then to full brightness is increased.

Some lught dimmers have also a setting to adjust the control voltage
range. 0-10V controlling is the most common way to do the controlling
of small dimmer systems, but there have been also other voltage levels
in use. If the dimmer has an adjustment which voltage range it takes,
it can be adjusted to work correctly with many different light control
desks.

The simplest form of the controlling is that the voltage directly controls
the phase when the triac condicts. This works, but is not the
best response from the control potentiometer to the dimemr module.
For this reasons differen manufactuers have developed many different
response curves from the control voltage to the dimmer output.
Here are some of the most common ones:

Square: The output power varies linearly with the input (square law ramp standardized by United States Illuminating Engineering Society). At setting of 50% you will see alight level of arounf 50% of maximum.

S curve: A modified form of Square with greater control in the centre of the range

True power: The output power varies linearly with the input voltage so that the lamp get 50% of it's nominal power on 50% setting (used more on industrial control than in light dimming)

Exponential ramp: Light output varies most in the contro range of 70% to 100%

Relay: The output switched to the full when the input exceeds 25% of the full control voltage (with some equipment the limit is 50%)

Nowadays some advanced commercial dimmers support many of those
control voltage response curves so that the user can set the dimemr to use
the mode which is the most convient for the user in the particular
application.

Phase controlling using microprocessor

If you want a digital control of light dimmer
you can use a simple microcontroller to do the phase controlling.
The microcontroller has to first read the dimmer setting value through
some interface (commercial digital dimmers use DMX512 interface).
typically the control value is 8 bit number where 0 means light
off and 255 that light is fully on.

The microcontroller can easily generate the necessary trigger
signal using following algorith:

Convert the light value to software loop count number

First wait for a zero crossing

Run a software loop which waits the necessary time till it time to trigger the TRIAC

Send a pulse to the TRIAC circuit to trigger the TRIAC to conduct

Software loop is quite simple method and useful when you know how long time it takes
to execute each microprocessor command. Another possibility is to utilize
microcontroller timers:

You can generate an interrupt at every zero crossings and every timer count.

At every zero crossing the microcontroller loads the delay value to the timer
ands starts it counting.

When the counter time has elapset it generates an interrupt. The timer interrupt routine
sends a trigger pulse to the TRIAC circuit.

Reverse phase control is a new way to do light dimming.
The idea in reverse phase controlling is to turn on then switching component
to conduct at at every zero crossing point and turn it off at the adjustable
position in the middle of the AC current phase. Tming of the turn-off point
then controls the power to the load. The waveform is exact reverse
of that is used in traditional light dimmers.

Because the switching component must be turned off at the middle
of the AC phase, traditional thyristors and TRIACs are not
suitable components. Possible components for this kind of
controlling would be transistors, FETs, IGBTs and GTO-thyristors.
Power MOSFETs are quite suitable components for this and
they have been used in some example dimmer circuits.

Reverse phase controlling has some advantages over traditional
dimmers in many dimmer applications. The manufactuers
of inverse phase dimmers adverstise their products to
be more efficent and less noisy. Using proper controlling
electronics it is possible to build a reverse phase dimmer without
any magnetics or vibrations caused by them.

Because turning on point is always exact at the zero phase there are
no huge current spikes and EMI caused by turn on. Using power MOSFETs it is
possible to make the turn-off rate relatively slot to achieve
quite operations in terms of EMI and acoustical or
incandescent lamp filament noise.

One old approach for dimming of lights is do it by using
variable transformer (Variac or similar brand) as a dimmer.
Some of these are made specifically for this
application - they'll fit into a double-size wall box (maybe even into
a single-size wall box if you get a small one) and will handle several
hundred watts. They're heavy and mechanically "stiff" (compared to a
triac dimmer) and not cheap - but they put out a nice, clean 60 Hz
sinewave (or very near to it) at all voltages, and don't add switching
noise.

Zero cross switching will minimize noise in switchign and
dimming. Unfortunately that appriach is not very practical
for lampi dimming. At 60 Hz line frequency,
you'd be limited to turning the lamp on
and off at discrete 120 Hz intervals.
You'd easily end up with a rather nasty 15-20 Hz flickering,
unless the dimmer-driver can do
some sort of dithering to spread out the flicker spectrum. I've never
seen a dimmer of this sort being used.

In some occasions a single diode can be to dim a light bulb when wired
in series with the lamp. The diode then passes only the positive or
negative half of the mains voltage to the light bulb. If you put a switch
in parallel with the diode, you end up having a dimmer wich has two
settings: full on and dimmed. Diode will indeed work on small
loads, but with larger loads the DC component this diode causes
is not good for the distribution transformers in the
electrical distribution system (will cause them them to heat up more
than in normal use).

NOTE: The following information is taken from the discussion
from sci.engr.electrical.compliance newsgroup discussion
at February-March 2000.
The facts have not been checked against any standard documents,
but I suspect that the information is quite much correct because
most of the writers of the articles where experts
on the field (for example John Woodgate) and the information
makes sense to me.

Harmonics

Mains harmonics are typically tested from mains frequency up to 2 kHz frequency
(2.4 kHz in 60 Hz countries).
Phase controlled dimmers up to 1 kW do not need to be tested for
harmonics. There is no point, because the harmonics are very predictable
and there is nothing much the designer can do to reduce them.

Professional (as defined in IEC/EN61000-3-2) dimmers over 1 kW up to
3680 W are also not subject to limits.

Dimmers above 3680 W, which are all professional, come under the future
IEC/EN61000-3-12, and it is still being discussed whether they need to
have an Rsce (as defined in IEC61000-3-4) limitation or not.

Conducted emissions

Light dimmers need to meet conducted emission standards.
Conducted emissions start at
9 kHz for some products and for dimmers the applicable standard for
those is CISPR15/EN55015. That standard is applicable to lighting
equipments and an accessory for a luminaire (like a light dimmer is).

There is no exception in CISPR15/EN55015 standard (which now applies, rather than
CISPR14/EN55014). Dimmers for household use need to meet Class B limits,
but Class A should be OK for professional dimmers.
The conducted emissions are mostly harmonics and can exist up
to megahertz frequecny region.

To meet the conducted emission limits is not very easy, especially for
professional dimmers. The choke hardly helps, because a typical
filtering self-resonates at around 100 kHz (higher for low-power household dimmers).
Above those frequencies the coil does not suppress the high frequency harmonics.
This means that it is often necessary to sprinkle
quite large (up to 1 uF) capacitors around the circuit to reduce the
emissions. In professional dimmers this demands that inductances in
the wiring be reduced to a minimum, otherwise the caps and wiring
inductances resonate and emissions go up instead of down.

A lot of manufacterers of professional dimmers ground thyristors heat
sink, effectivly coupling RF noise into the earth lead. This will reduce
the radiated emissions and there might be safety considerations
to do that. The downside of the RF (harmonics) coupled to
ground wire is that in some cases
the inductance of the earth lead is so high that the appliance case
carries a noticeable voltage.